Synchronous filter



Dec. 26, 1950 G. R. CLARK 2,535,940

SYNCHRONOUS FILTER Filed April 24, 1946 A TTOIPIVEY Patented Dec. 26, 1950 UNITED STATES PATENT OFFICE sYNcnaoNoUs FILTER Gilbert R. Clark, Nutley, N. J., assignor to Federal Telephone and Radio Corporation, New York, N. Y., a corporation of Delaware Application April 24, 1946, Serial No. 664,483

13 Claims. l

This invention relates to electrical Wave filter systems and more particularly to lter systems capable of providing relatively sharp filtering at low frequencies where alternating current output is desired.

In my copending application, Serial Number 591,894, filed May 4, 19.45, now abandoned, I have disclosed a ltering system which is based on the accumulative effect of stored oscillating energy, which energy is` delivered to the system at a given frequency and' at a constant phase. The system described provides for a selection of harmonically related frequencies. lReasons were set forththerein whichV make it desirable in communication systems to obtain systems and methods for achieving a high degree, of selectivity, such systems being designed to store energy in a static state instead of dynamically, as has been the case in previous types of filters. Sys.- tems of thisA type are basically synchronous, that is, their operation `depends on the, storage Vof Wave energy in synchronism with the frequency it is desired to filter out.

The present lter system has basic similarity to the one disclosed in the above-named application insofar as the storage of oscillating energy is static instead of dynamic; in that it requires recurrent switching operation; .and thatw the switching rate is related to the frequencies el,` frequency which it is desired to. select. However, while the harmonic lter system of the above,- named copending application, as its name implieS. is used to select or isolate a, series of harmonic frequencies, the present system filters a single given frequency only.

It is` an object f this invention to. provide a lter system which is highly selective in respect t0 a single rgiven frequency in the audio. frequeney band- Y It is a further object of the invention to provide an electro-mechanicalrotary Wave filterv for.

the selection of a given frequency te the exclusion of other frequencies.

It is a still further Object to provide a rotary, wave filter of the type defined above which prevides an alternating current output.

In accordance with certain` features, of the invention I provide a plurality of separate electric energy Storage means, preferably electr@- static, and means for applying and taking-off cyclically and in a predetermined order energy at a given rate With respect to these storage means.l 'Ihe number of the storage means is such asto make them responsive to a wave of a Single frequency only. the.. Seeed of the Cycle of energy application and take-off being the determinate of the respective selected frequencies.l In practice, three storage elements, such as condensers, are used in such a wayv as to each receive energy for'storage thereon during a 120 portion of the applied Wave. The effect of the system is to'operate as a synchronous rectifier for given portions of the Wave which are corbincd into a resulting alternating current type Wave, the output wave form having a somewhat square shaped appearance. The output wave form, however, may be filtered back into a sinusoidal Wave form quite readily.

Other features and advantages not particularly described will be apparent and av better understanding of my invention may be had from a consideration of the following description and the appended claims. In the drawings,

Fig. 1 represents in a block diagrammatic form a rotary electrofmechanical filter system in aeordance with the invention;

Fig. 2 illustrates input and output wave forms of the system; and

Fig. 3 illustrates in graph form input and output Wave forms of an alternative phase condition of the system.

Referring to Fig. 1, a source of oscillating electrical energy. having various Wave forms is indicated as a coupling device. shown in the form of a transformer I which may be connected to `any desired source. of audio frequencies, the source itself not being indicated in the drawings. The oscillating energy from the coupling device l is applied to a rotary selector 2, which serves to select the desired single frequency. The. energy at this given frequency is then applied over an output coupling transformer 3 to a utilization circuit 4. l

'I he rotary selector 2 comprises an inputr contact ring 5 which is connected to the input coupling transformer l over an input resistance 6. An output ring l, concentric with the ring 5, serves to conduct energy to ther output coupling transformer 3 over a comparatively large output resistance 1A. The energy is cyclically stored on three condensers 8, 9 and l0 which are connected between ground and circular conductor rings Il, l2 and i3 respectively. A series of repeated groups of conductive segments as at i4, I5 and I6, are connected to rings ll, I2 and i3 in such a way as to successively and cyclically couple thereto. three condensers 8, 9 and i6 around the circumference of the rotary selector 2. The selector is provided with a rotary input brush comprised of a rotary arm ll which has.

'mounted thereon a brush I8 making contact with the input ring E and a second brush I9, arranged to contact the various segments ifi, I5, I6 etc., a conducting path IiA being provided between the brushes I8 and I9. Thus energy from transformer I is successively applied to condensers 8, 9 and Iii to build up charges therein upon rotation of arm I'I. Similarly, for the pur pose of obtaining output energy from the filter, a rotating arm 2i] has been provided which has mounted thereon an output brush 2i and a segment contacting brush 22 and conducting path 22A. The two arms Il' and 2u are mounted for rotation about a shaft 2t which is driven by a motor 24 subject to a control in speed by a control device 25. The two arms i7 and 2li may be arranged to form any angle therebetween although 180 is preferable because of the mechanical balance thereby provided.

The Wave form illustrated in graph a of Fig. 2 is representative of a wave and frequency of the type which has been isolated by the iilter, the effect of the combination of the segments and of the three condensers being to divide the incoming frequency into threeequal sections of 120 each, as indicated. Each of the three condensers acts as a synchronous rectier forl the component of the wave, to which it is periodically subjected through the brush system IB and I9, represented by the cyclic repetition rate at which the input brush I'I contacts repeatedly input segments I4, I and I6. By controlling the phase of motor 24 the selected frequency component of the input wave may be chosen. Since the output resistance 'IA in the take-off circuit represented by the brush arm 2l and 22 is relatively high, the condensers will not be discharged, the eiiect rather being that of a reading of the respec tive condenser voltages which is transmitted to the utilization circuit. Any applied frequency which is multiple of the brush speed in revolutions per second applies the same polarity and magnitude of potential to each of the condensers during each revolution. All other frequencies will tend to both charge and discharge the condensers from time to time. As the signal input ceases, the build-up voltages `on the ccndensers are gradually dissipated to ground over input resistance 6 if the brushes continue to rotate, at a rate determined by the site of the resistance.

The resulting take-olf wave form as obtained in the coupling transformer is shown in graph b of Fig. 2, when the phase relation between the rotating brush system and the desired frequency is maintained as shown in graph a of Fig. 2. Should the rotary brush system be maintained for instance at a phase difering by 39 from that of Fig. 2, as suggested in graph a of Fig. 3, the resulting take-off Wave -forni would take the shape as in graph b (Fig. 3).

In the design of the filter, it is preferred that, if a single frequency only is desired, the nunmber of condensers employed be three, arranged in the manner illustrated. Although a number of condensers other than three is within the present scope, certain difficulties arise. Thus, if two condensers were used, the possibility would occur that under certain phase conditions the charges on the two condensers would cancel out and thus not making available any output for the desired frequency. If more than three condensers are employed, certain harmonics of the desired fundamental frequency can build up energy in the storage condensers under favorable phase conditions. However, it is understood that any number other than three storage numbers may be employed, if desired.

Ordinarily, while only the frequency of the resulting alternations is desired, rather than a specific wave form, the resulting rectangular shape of the Wave form as suggested in graph b of Figs. 2 and 3, may be easily filtered back into a sinusoidal form, if required, by the use of a tuned circuit or low pass iilter following the output transformer 3. It will be realized that each of the three condensers is usually being charged exponentially through the resistor 5 until, after a given number of charging contacts have been made, a resultant constant value of the charges thereon will have become established as shown in the graphs b.

Although I have described the rotary iilter for a single frequency as employing electrostatic energy storing means it will be apparent that other energy storing means may be employed analogously.

While the above is a description of the principles or" this invention in connection with speciiic apparatus, it is to be clearly understood that this description is made only by Way of example and not as a limitation on the scope of this invention as set forth in the objects and the accompanying claims.

I claim:

l. A rotary wave iilter for the selection of a single frequency comprising three energy storing eondensers grounded on one side each forming a submultiple energy storage means, a circular conductor for each of said condensers connected to the other side thereof, said circular conductors being disposed concentrically with respect to one another, a plurality of contact members conductively associated with and radially distributed with respect to each of said conductors, the contact members of said conductors being arranged in succession and forming cyclic-radially occurring groups; a circular input contact slip ring; a circular take-off contact slip ring coaxial with said first ring; rotary input energy transfer kbrush means for conductively bridging said input slip ring and one of said contact members; and rotary take-off energy transfer brush means for conduc tively bridging said take-off slip ring and another of said contact members, said intake and said take-off brush means being angularly displaced to simultaneously contact the contact members associated with the same circular conductor in successive contact member groups.

2. A rotary wave filter according to claim 1` further including an input circuit comprising a coupling transformer and an input resistance connected t0 said input slip ring.

3. A rotary wave filter according to claim 1 further including an output circuit comprising a relatively high output resistance connected to said take-off slip ring and an output coupling transformer.

4. A rotary wave filter according to claim 1 further including means for simultaneously rotating said input and take-off brushes about the center of the circles formed by said slip rings and said contact members respectively.

5. A filter according to claim 1, in which said condensers comprise three in number.

6. A rotary wave lter for the selection of a single frequency from a source of oscillatory energy of more than one frequency comprising three energy storage means, a iirst movable contact for applying energy successively and cyclically from a given source to each of said storage in synchronism with the recurring portions of a wave of a given frequency, a second movable contact for successively obtaining signals fro-m each of said storage means in synchronism with said given frequency, an annular conductor connected to each of said energy storage means, each said annular conductor having a plurality of contact members connected thereto and circumferentially arranged, said contact members of all said storage means being disposed in successively recurring groups with respect to one another, said first and second movable contacts being mounted to contact said contact members.

7. A illter according to claim 6 in jfwhich an annular input contact slip ring is mounted coaxially with said annular conductors and a rotary input energy transfer brush is arranged conductively to bridge said input slip ring'and one of said contact members.

8. A filter according to claim 7 in which a rotary take-off contact slip ring is mounted coaxially with said annular conductors and rotary take-01T energy transfer brush is arranged conductively to bridge said take-off slip ring and one of said contact members.

9. A filter according to claim 8 in which said input and take-off brushes are angularly displaced to contact simultaneously the contact members associated with the same annular conductor in successive contact member groups.

10. A filter acording to claim 9 in which said storage means comprise separate electrostatic condensers.

11. A filter according to claim 10 in which a coupling transformer and an input resistance are connected to said input slip ring.

l2. A lter according to claim 11 in which a relatively high output resistance in connected to said take-off slip ring and an output coupling transformer is connected to said resistance.

13. A filter according to claim 12 in which a motor is connected to said input and take-off brushes for simultaneously rotating themabout the center of the circles formed by said slip rings and said contact members and said annular conductors.

GILBERT R. CLARK.

REFERENCES CITED The following references are of record in the file of this patent: i

UNITED STATES PATENTS Number Name Date 1,851,090 Fetter June 21, 1927 1,851,092 Fetter June 21, 1927 2,006,582 Callahan et al Feb. 25. 1933 2,430,038 Wertz Nov. 4, 194'? 

